Mobile communication systems that perform communication using radio communication channels are currently coming into widespread use globally. In such radio communication systems, multiple base stations (BSs) are arranged in a service area and a mobile station (user equipment (UE)) communicates with another communication device (communication terminal) via one of the base stations. Each of the base stations has an area referred to as a cell within which radio communication can be performed between a mobile station and the base station.
However, when a mobile station is located close to the boundary of a cell of a base station, the communication state between the mobile station and the base station becomes worse and, in the worst case, establishment of communication is difficult. Furthermore, due to landscape constraints or due to being shielded by buildings, the communication state between a mobile station and a base station may possibly become worse or the establishment of communication may possibly be difficult.
Accordingly, in order to establish stable communication between a mobile station and a base station by overcoming such disadvantages, relay devices (relay stations) are arranged. The relay devices relay data transmission from a base station to a mobile station and also relay signal transmission from a mobile station to a base station. Specifically, instead of either a mobile station or a base station directly receiving a signal from the other, the mobile station or the base station receives, by using a relay device, a signal transmitted from the relay device. Thus, communication can be established between the base station and the mobile station.
Furthermore, in radio communication systems, in order to implement high-speed and high-volume radio communication, improvements to the currently used radio communication systems or the next-generation mobile communication systems are being actively discussed at standardization conferences related to, for example, the 3rd Generation Partnership Project (3GPP). In the 3GPP project, studies have been conducted on LTE advanced, which is an expansion of the Long Term Evolution (LTE) system that is one of the radio communication systems.
In LTE advanced, studied as part of the 3GPP project, the introduction of two types of relay devices is being studied: a relay device that does not transmit, to a mobile station, a signal used to specify the relay device and a relay device that transmits, to a mobile station, a signal used to specify the relay device. In the following, a description will be given of a relay device that does not transmit, to a mobile station, a signal used to specify the relay device.
A mobile station does not recognize a relay device as a relay device unless the relay device transmits a signal used to specify the relay device to a mobile station. Specifically, the mobile station does not recognize that a signal is being relayed even though a relay device is relaying the signal that is being transmitted between the mobile station and the base station. Accordingly, if a relay device that is not recognized as a relay device by a mobile station is used, the mobile station operates as if it is directly communicating with a base station.
In radio communication systems that use relay devices that are not recognized as a relay device by mobile stations, if a relay device relays user data, a mobile station receives the user data from the relay device. At this time, in some cases, the base station transmits, to the mobile station, data having the same content as the user data transmitted by the relay device. In such a case, the base station transmits the data such that the data transmitted to the mobile station by the base station is received when the mobile station receives the user data from the relay device. In contrast, if the signal transmitted by the base station is a common control signal containing information on the radio communication system or a control signal attached to the user data, the mobile station receives these signals from the base station. The control signal attached to the user data contains information needed to demodulate or decode the user data.
In radio communication systems that use relay devices, when communicating with a mobile station, there is a need for selecting which relay device relays which user data signal to be transmitted to a mobile station located in the cell of a base station. Accordingly, if a mobile station does not recognize the presence of a relay device, a base station selects a combination of a mobile station and a relay device. In the following, a method for selecting such a combination will be described. Each relay device receives a signal that is transmitted to the base station by a mobile station that makes a request to establish communication. Then, each relay device notifies the base station of received power of the signal transmitted from the mobile station that makes a request to establish communication. Then, by using the reception quality acquired from each relay device, the base station selects a relay device that relays the signal transmitted from the mobile station that makes a request to establish communication.
In the following, this conventional technology will be more specifically described with reference to FIG. 11. FIG. 11 is a sequence diagram illustrating a process for establishing communication between a mobile station and a base station via a relay device by using the conventional technology. The downward direction of the vertical lines in the plane of the drawing illustrated in FIG. 11 represents the elapse of time. Each of the vertical lines illustrates the operation in the chronological order it is performed by the device indicated at the top of the vertical line. In this example, a description will be given of a case in which the reception quality of the signal transmitted from a mobile station 91 to a base station 93 is poor at the base station 93 and the state of the radio communication between the mobile station 91 and the base station 93 is inferior. In the following, a description will be given by using received power as the reception quality.
The mobile station 91 needs to perform the synchronization when the mobile station 91 radioly receives a signal from the base station 93. Accordingly, the mobile station 91 needs to transmit a synchronization signal to the base station 93 in order to perform synchronization. For example, the synchronization signals can be used as different preamble signals in each base station 93.
The base station 93 notifies the mobile station 91 of the time at which the mobile station 91 transmits the synchronization signal (Step S901). This signal is a control signal and is directly received by the mobile station 91 without passing through a relay device 92.
The mobile station 91 arbitrarily selects a single code from among previously determined multiple codes (or a sequence) and creates a synchronization signal by using the code. For example, the code mentioned here indicates a code used by a computer to identify characters or figures. In the following, a description will be given of a case in which the mobile station 91 uses a code as a synchronization signal. In this example, it is assumed that each code has a number representing a mobile station itself. Then, the mobile station 91 transmits, to the base station 93, a synchronization signal at the time specified by the base station 93 (Step S902). At this time, the synchronization signal is also directly transmitted from the mobile station 91 to the base station 93 without passing through the relay device 92. At this time, the relay device 92 receives the synchronization signal that is transmitted from the mobile station 91 to the base station 93 (Step S903). However, the relay device 92 does not transfer the synchronization signal to the base station 93.
The relay device 92 acquires a code number from the synchronization signal. Furthermore, the relay device 92 measures the received power of the synchronization signal. Then, the relay device 92 transmits the code number of the synchronization signal and the received power to the base station 93 (Step S904).
The base station 93 receives the synchronization signal from the mobile station 91. Then, the base station 93 acquires the code number from the synchronization signal. Furthermore, the base station 93 measures the received power of the synchronization signal. Furthermore, the base station 93 acquires, from the relay device 92, the code number of the synchronization signal and the received power received by the relay device 92. Then, the base station 93 compares information on the synchronization signal received by the mobile station 91 with information on the synchronization signal acquired from the relay device 92 and determines whether the base station 93 allows the relay device 92 to relay communication to the mobile station 91 (Step S905). In this example, a description will be given of a case in which the base station 93 determines that it does allow the relay device 92 to relay communication to the mobile station 91.
The base station 93 notifies the mobile station 91 of the code number acquired from the synchronization signal (Step S906). At this time, in practice, the base station 93 notifies all of the mobile stations, which are present in the cell of the base station 93, of the code number.
The mobile station 91 determines whether the code number of the synchronization signal transmitted by the mobile station 91 matches the code number notified by the base station 93 (Step S907). If the mobile station 91 determines that the code numbers match, the mobile station 91 transmits, to the base station 93, a response signal indicating that the code number has been confirmed (Step S908).
The base station 93 receives the response signal transmitted from the mobile station 91 and specifies the mobile station 91 that has transmitted the synchronization signal containing the acquired code number. Then, the base station 93 instructs the relay device 92 to relay communication to the mobile station 91. Accordingly, the base station 93 establishes radio communication with the mobile station 91 via the relay device 92 (Step S909).
Furthermore, the following conventional technologies have been proposed as a technology for performing radio communication via a relay device. First, there is a conventional technology in which a mobile station determines, on the basis of the received power of a signal received from a base station, whether the mobile station acts as a relay device. Furthermore, there is also a conventional technology in which, in order to avoid limitation of the radio resources (frequency) allocated to a relay device or a mobile station, a base station notifies a relay device of the carrier to be used and makes the carrier used by each relay device different. Furthermore, there is also a conventional technology for monitoring the propagation path by a relay device notifying a base station of the received power of signals received from multiple mobile stations.
Patent Document 1: Japanese Laid-open Patent Publication No. 2008-048202
Patent Document 2: Japanese Laid-open Patent Publication No. 2009-231990
Patent Document 3: Japanese Laid-open Patent Publication No. 2005-252677
Patent Document 4: Japanese Laid-open Patent Publication No. 2000-193697
However, in the conventional technology described with reference to FIG. 11, in order to specify a mobile station that transmits a code number acquired by a relay device and a base station, after the mobile station has transmitted the code number, signal transmission needs to be repeated between the base station and the mobile station. Accordingly, it takes a long time to establish communication between the base station and the mobile station via the relay device.
Furthermore, to reduce the time for the establishment of communication between the base station and the mobile station via the relay device, the base station previously needs to designate a code number that is transmitted by the mobile station that is the target for the establishment of communication. However, the relay device does not recognize the code number designated by the relay device if the base station simply specifies the code number to the mobile station. Accordingly, the relay device also receives a radio signal containing another code number that is output from a mobile station that is other than the mobile station corresponding to the target for the establishment of communication in the cell and transmits the code number and the received power to the base station. For example, in the 3rd Generation (3G) system, 64 code numbers are present in a single cell, and a relay device needs to transmit a maximum of 64 code numbers and received power to the base station. Under such circumstances, the load on the relay device at the time communication is established becomes high, which increases the electrical power consumed by the relay device.